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Inorg. Chem. 2001, 40, 1466-1476
Time-Resolved Optical and Infrared Spectral Studies of Intermediates Generated by Photolysis of trans-RhCl(CO)(PR3)2. Roles Played in the Photocatalytic Activation of Hydrocarbons1 Jon S. Bridgewater,† Thomas L. Netzel,‡ Jon R. Schoonover,§ Steven M. Massick,† and Peter C. Ford*,† Department of Chemistry, University of California, Santa Barbara, California 93106, Department of Chemistry, Georgia State University, Atlanta, Georgia, and Polymers and Coatings Group, Material Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 ReceiVed NoVember 16, 2000
Described are picosecond and nanosecond time-resolved optical (TRO) spectral and nanosecond time-resolved infrared (TRIR) spectral studies of intermediates generated when the rhodium(I) complexes trans-RhCl(CO)L2 (L ) PPh3 (I), P(p-tolyl)3 (II), or PMe3 (III)) are subjected to photoexcitation. Each of these species, which are precursors in the photocatalytic activation of hydrocarbons, undergoes CO labilization to form an intermediate concluded to be the solvated complex RhCl(Sol)L2 (Ai). The picosecond studies demonstrate that an initial transient is formed promptly (50 ms) to be probed with the current setup owing to diffusion out of the excitation volume. However, since the spectrum of a benzene solution of I did not change after repetitive laser flashes (1000 shots at 2 Hz), CI must also eventually decay to re-form I. This is consistent with published results stating that little net photochemistry was observed after continuous photolysis under anaerobic conditions.25 CI was not observed in the TRO studies under added CO; thus AI is the likely precursor to CI perhaps via dimerization or reaction with I to form dinuclear complexes.11,26 Flash photolysis (355 nm) of a trans-RhCl(CO)(PPh3)2 solution (0.2 mM in benzene) containing both PPh3 (23 mM) and CO (0.72 mM) leads to formation of a transient absorbance at 410 nm largely due to AI. The decay of this transient was modeled with a single exponential to give kobs ) (1.3 ( 0.2) × 106 s-1. If second-order PPh3 trapping of AI to give RhCl(PPh3)3 is competitive with CO trapping to give I, then kobs ) k PPh3[PPh3] + k CO[CO], from which can be calculated the value kPPh3 ) 5.6 ( 0.6 × 107 M-1 s-1 as an upper limit.27 This ignores contributions of dimerization and oxidative addition pathways to the decay of transient AI, a valid approximation since these pathways appear to be but a minor part of the total decay ( 4 mM. Thus, AII has the behavior expected for the tricoordinate species “RhClL2” and BII appears to be largely formed from AII in competition with CO trapping. The disappearance of BII could be fit to a monoexponential decay with kobs ) (1.6 ( 0.4) × 103 s-1, independent of [CO]. Supplemental Figure S-5
1472 Inorganic Chemistry, Vol. 40, No. 7, 2001 displays the spectrum of II and transient spectra recorded 50 ns (AII) and 50 µs (BII plus CII) following flash photolysis of trans-RhCl(CO)(P(p-tolyl)3)2 in benzene under CO (6.7 mM). Solvent effects were qualitatively the same as seen above for I. The kCO values obtained from linear plots of kobs vs [CO] were (4.6 ( 0.4) × 108 M-1 s-1 in CH2Cl2 and (6.2 ( 0.5) × 108 M-1 s-1 in cyclohexane but much smaller, 1.3 ( 0.4 × 107 M-1 s-1, in THF (Table 1). The CO-independent terms (the intercepts) were (1.0 ( 0.5) × 105 s-1 and (2.0 ( 0.4) × 104 s-1 in dichloromethane and THF. The decay of BII displayed a rate constant of 3 × 103 s-1 in CH2Cl2, but the low signal/ noise ratio prevented following this in cyclohexane. TRO Studies of II Using Picosecond Excitation. The picosecond studies in deaerated THF also gave results closely analogous to those for I. Flash photolysis led to prompt (50 ms) transient BIII, which was too long-lived to follow its decay. Analogous behavior was seen when the transient bleach was monitored at 380 nm. Under argon, the decay of AIII was of mixed order, but under added CO this was pseudo-first-order. A plot of kobs vs [CO] (0.050.7 mM) was linear, giving kCO ) (1.2 ( 0.2) × 109 M-1 s-1 with an intercept of (1.5 ( 0.8) × 105 s-1. Similar transient kinetics were observed in cyclohexane, CH2Cl2, and THF, and the respective kCO values were (6.5 ( 0.8) × 108, (1.3 ( 0.2) × 109, and (3.7 ( 0.2) × 108 M-1 s-1 (Table 1). (The respective intercepts were (5.1 ( 1.0) × 105, (3.5 ( 2.0) × 105, (1.7 ( 0.4) × 104 s-1.) Notably, kCO in THF is only a factor of 3 smaller than in benzene, in contrast to the observations with AI and AII, where solvent effects are much larger. Figure 8 illustrates the TRO spectra immediately (0-400 ns) after and 18-20 µs after 355 nm flash photolysis of III in argondeaerated benzene containing 0.42 mM PMe3. The initial spectrum is that of AIII. This undergoes exponential decay to reveal a new long-lived transient with spectral properties consistent with formation of RhCl(PMe3)3 (eq 3). A plot of kobs vs [PMe3] (0.2 to 1.0 mM) is linear with a slope (kPMe3)
Bridgewater et al.
Figure 8. Transient spectra from the flash photolysis of trans-RhCl(CO)(PMe3)2 in the presence of added PMe3.
of (1.1 ( 0.2) × 109 M-1 s-1 and an intercept of (1.5 ( 0.5) × 104 s-1. kPMe3
AIII + PMe3 98 RhCl(PMe3)3
(3)
TRO Studies of III Using Picosecond Excitation. The picosecond flash studies of III in deaerated THF revealed a more complicated scenario than seen for I and II. Again, a transient is formed promptly ( 300 nm, the resulting FTIR spectrum showed bleaching of the1962 cm-1 νCO band for the parent and the appearance of a broad band at 2070 cm-1 with a shoulder about half as intense at 2050 cm-1 (supplemental Figure S-7). Other bands, less than onefourth the intensity of the 2070 cm-1 band, were visible at 2130 and 2018 cm-1. These spectral changes displayed an indefinitely long lifetime at 213 K. The new bands are consistent with the νCO values expected for Rh(III)-coordinated CO. Rhodiumhydride stretches νRhH of related compounds fall in the same frequency range.28 The band at 2130 cm-1 is coincident with that expected for free CO, but the relative intensity appears too large for that species alone.29 When the photolysis was carried out in perdeuterio solvent, the bleach at 1962 cm-1 was accompanied by appearance of only one new absorption, a strong νCO band at 2050 cm-1, broad in a manner suggesting two overlapping bands. This suggests that the absorbances appearing for the photoproduct in perprotio benzene at 2130 (28) (a) Kaesz, H. D.; Saillant, R. B. Chem. ReV. 1972, 72, 231-281. (b) Intille, G. M. Inorg. Chem. 1972, 11, 695-702.
Photocatalytic Activation of Hydrocarbons and 2018 cm-1 are νRhH bands, while the two seen at 2070 and 2050 cm-1 represent νCO bands for a pair of monocarbonyl products. This is in agreement with the earlier NMR studies which concluded that two Rh(III) isomers (X and Y) were formed in comparable concentrations upon photolysis of III under analogous low-temperature conditions.7d,9a,30 The modest frequency shift(s) for the νCO bands in C6D6 relative to the perprotio medium may reflect coupling to a Rh-H of comparable frequency in the latter system.31
TRIR Studies of III. Temporal IR absorbance changes following 355 nm excitation of III in benzene were monitored using a tunable single-frequency probe source set at 2070 cm-1. Two processes were evident, prompt rise within the time constant of the instrument (
